Free-radical polymerization or cross linking reactors are fed with free-radical initiators, which may be organic peroxides, azo compounds or carbon-carbon initiators from the class of hexasubstituted ethanes (Encyclopaedia of Chemical Technology, Kirk-Othmer, Fourth Edition, Vol. 14, 1996, pages 436-53). In the majority of cases they are organic peroxides, supplied by producers of organic peroxides, and generally require a step of transport and of storage.
Organic peroxides are unstable compounds which are able to undergo thermal decomposition at higher or lower temperatures, depending on their structure. Their preparation, transport and storage consequently necessitate very substantial precautions in order to prevent any accident in the course of their handling.
In order to avoid problems relating to the regulations associated with the transport of these dangerous products, and to limit their storage, it is sometimes recommended to produce the organic peroxide directly at the site of its use.
Organic peroxides are generally prepared in conventional open reactors by discontinuous (batch) processes, this type of process being particularly suitable for the secure production of moderate amounts of peroxides (Encyclopaedia of Chemical Technology, Kirk-Othmer, Fourth Edition, Vol. 18, 1996, pages 292-293). However, not all organic peroxides can be prepared according to conventional batch conditions, owing to their instability. Some can be prepared only in solution, or must then be stabilized in the form of an emulsion or by the addition of a stabilizer. This is the case in particular for highly reactive peroxides, in other words for peroxides whose 10-hour half-life temperature is relatively low.
These drawbacks can be circumvented by producing the peroxides in situ, in other words in the reactor in which the polymerization or cross-linking is carried out. Mention may be made, for example, of U.S. Pat. No. 5,700,856, which describes the preparation of ketone peroxides in a system which contains the unsaturated polyester resin and the cross-linking accelerator.
However, this type of in situ manufacturing process for the organic peroxide does not allow the peroxide feed to the polymerization reactors to be automated. A major drawback is the lack of precision with regard to the amounts of peroxide effectively employed for the polymerization, and the need to precede each polymerization cycle by the in situ synthesis of the initiator. The productivity of the plant is limited as a result.
Proposals have also already been made to prepare the organic peroxides just adjacent to the polymerization or cross-linking reactor (ex situ synthesis).
In document FR 2253760 a peroxydicarbonate is prepared from an alkyl haloformate and an inorganic peroxide compound in the presence of water and a volatile, water-immiscible solvent, immediately before the polymerization. The whole of the reaction mixture thus obtained (organic phase and aqueous phase) is then introduced into the polymerization reactor, which is subsequently charged for the purpose of polymerization. This process does allow the organic peroxide feed to the reactors to be automated, but requires the production of an amount of initiator which is just sufficient, immediately prior to the polymerization. Furthermore, it exhibits drawbacks from the standpoint of the quality of the polymerization and the polymers obtained.
International application WO 97/27229 describes a process for manufacturing a dialkyl peroxydicarbonate solution which is particularly suitable for implementation in a process of aqueous suspension polymerization of vinyl chloride. The solutions of dialkyl peroxydicarbonate in a liquid, water-insoluble dialkyl alkanedicarboxylate can be prepared in advance in sufficient amount to feed a number of polymerization reactors, but are stored at low temperature so as not to give rise to any hazard. They may be introduced in whole or in part after the beginning of the polymerization, which allows the feed to the reactors to be automated. However, the use of the organic peroxide in the form of a solution in an ester may be detrimental to the polymerization kinetics and to the general properties of the polymers produced. Furthermore, the preparation of dialkyl peroxydicarbonate solutions involves a step of separation of the dialkyl peroxydicarbonate prepared in water, by extraction using the ester.
In international application WO 03/074573, diacyl peroxides are prepared in an aqueous medium by a peroxidation process and are transferred to a polymerization reactor within a time period of between 2 hours and 168 hours after their preparation. The reactor in which the peroxidation reaction takes place may also be connected directly to the polymerization reactor. However, it is not possible to operate in this way for highly reactive organic peroxides in respect of which the refrigeration and explosion danger demands are stringent. Furthermore, the use of a peroxide in aqueous solution is out of the question for some polymerization processes, such as bulk processes for polystyrene or polyethylene, or for the cross linking of polyester resins or polyolefins.
International application WO 05/075419 describes a multi-step process for preparing organic peroxides such as peroxydicarbonates, diacyl peroxides or peroxyesters wherein the reactants, such as acid chlorides or chloroformates, are prepared in situ from phosgene, which is also generated in situ. The peroxides thus obtained are then used directly in a polymerization reaction.